Hydrocarbons (oil, natural gas, etc.) are obtained from a subterranean geologic formation (i.e., a "reservoir") by drilling a well that penetrates the hydrocarbon-bearing formation. Hydrocarbon producers perform stimulation techniques to increase the net permeability of the reservoir. Stimulation techniques include: (1) injecting chemicals into the wellbore to react with and dissolve the damage (e.g., wellbore scaling); (2) injecting chemicals through the wellbore and into the formation to react with and dissolve small portions of the formation to create alternative flowpaths for the hydrocarbon (thus rather than removing the damage, redirecting the migrating oil around the damage); or (3) injecting chemicals through the wellbore and into the formation at pressures sufficient to actually fracture the formation, thereby creating a large flow channel though which hydrocarbon can more readily move from the formation and into the wellbore. The present Invention is directed primarily to the first and second of these three processes, although it can also be applied in the third process.
Thus, the present Invention relates to methods to enhance the productivity of hydrocarbon wells (e.g., oil and gas wells) by creating alternate flowpaths by dissolving small portions of the formation, or by removing (by dissolution) near-wellbore formation damage and scaling. Generally speaking, acids or acid-based fluids are useful for this purpose due to their ability to dissolve both formation minerals and contaminants (e.g., drilling fluid coating the wellbore or that has penetrated the formation) which were introduced into the wellbore/formation during drilling or remedial operations. The most common agents used in acid treatment of wells are mineral acids such as hydrochloric (HCl) and/or hydrofluoric (HF) acid.
At present, acid treatments are plagued by three serious limitations: (1) radial penetration; (2) axial distribution; and (3) corrosion of the pumping and well bore tubing. Limitations on radial penetration are caused by the fact that as soon as the acid, particularly mineral acid, is introduced into the formation or wellbore it reacts very quickly with the formation matrix and/or the wellbore scaling. Generally, the dissolution is so rapid that the injected acid is essentially spent by the time it reaches a few inches beyond the wellbore. Organic acids (e.g., formic acid, acetic acid) are sometimes used to address limitations on radial penetration since organic acids react more slowly than mineral acids. Sometimes, retarded acid systems, which use techniques such as gelling the acid, oil-wetting the formation, or emulsifying the acid with an oil, are used. Each alternative, however, has associated drawbacks and is an imperfect solution to limited radial penetration.
The second limitation of acid treatments, axial distribution, refers to the general desirability to limit the movement of the acid solution axially, so that it does riot intrude upon other zones in the subterranean formation, in particular, water-saturated zones. Conventional mineral acid treatment (e.g., HCl) has very high miscibility and the potential for undesirable migration of the HCl-based fluid into a water-saturated zone is a concern. Low miscibility acid fluids are desirable to minimize fluid migration away from its intended target (i.e., the desired hydrocarbon flowpath, or the damaged region).
Another ubiquitous problem with acid treatments is the corrosion of the pumping equipment and well tubings and casings, caused by contact with the acid (worse in the case of more concentrated solutions of mineral acids). Conventional acid treatments require the addition of a corrosion inhibitor; however, this can significantly increase the cost of a matrix acidizing treatment.
A related problem associated with acid treatments is iron precipitation, especially in sour wells (i.e., wells in which the oil has a relatively high sulfur content). Iron sulfide scale tends to form in boreholes, tubulars, and/or formations, especially in sour wells. The acid used to treat the well can dissolve the iron sulfide, but in the process; hydrogen sulfide is generated. H.sub.2 S is toxic and stimulates corrosion. In addition, the dissolved iron will tend to precipitate, in the form of ferric hydroxide or ferrous sulfide, as the acid in the treatment fluid becomes spent (i.e., fully reacted) and the pH of the fluid increases. Such precipitation of iron is highly undesirable because of the damage it can do to the permeability of the formation. Therefore, acid treatment fluids often contain additives to minimize iron precipitation and H.sub.2 S evolution, for example by sequestering the Fe ions in solution, or by reducing ferric ions to the more soluble ferrous form of iron.
U.S. Pat. No. 4,220,550, Composition and Method for Removing Sulfide-Containing Scale from Metal Surfaces, suggests the use of an aldehyde dispersed in acid to prevent the evolution of H.sub.2 S when removing sulfide-containing scale from metal surfaces. Examples of aldehydes disclosed as being suitable for this use include formaldehyde, acetaldehyde, and glyoxal. Unfortunately, formaldehyde has been listed as a suspected carcinogen. In addition, formaldehyde can react with HCl to form chloromethyl ethers which are known human carcinogens. Glyoxal has been used as a replacement for formaldehyde, but it is relatively expensive.
U.S. Pat. No. 4,289,639, Method and Composition for Removing Sulfide-Containing Scale from Metal Surfaces, discloses aqueous cleaning compositions for removing sulfide-containing scale from metal surfaces. The cleaning composition includes a nonoxidizing acid, such as HCl, and glyoxylic acid. The latter component is present in an amount sufficient to substantially prevent evolution of H.sub.2 S.
U.S. Pat. No. 4,734,259, Mixtures of .alpha.,.beta.-Unsaturated Aldehydes and Surface Active Agents Used as Corrosion Inhibitors in Aqueous Fluids, suggests that in acidizing well treatments, corrosion can be inhibited by including .alpha.,.beta.-unsaturated aldehydes and a surfactant in the acid treatment fluid. Examples of .alpha.,.beta.-unsaturated aldehyde disclosed as being useful for this purpose include cinnamaldehyde and certain derivatives thereof.
U.S. Pat. No. 4,888,121, Compositions and Method for Controlling Precipitation When Acidizing Sour Wells, discloses an acidizing composition that includes an acid such as HCl; an iron sequestering agent such as citric acid, ethylenediaminetetraacetic acid (EDTA), or nitrilotriacetic acid (NTA); and a sulfide modifier such as formaldehyde. This composition is stated to inhibit precipitation of ferric hydroxide, ferrous sulfide, and free sulfur, during the well acidizing treatment.
Although the above treatment fluids can help control iron precipitation, in some situations effective control would require the use of so much material that the treatment cost would become excessive. This would be especially true in wells with very heavy FeS deposits.
As evidenced by the references cited above, numerous techniques have been proposed to control acid corrosion and control the ferrous sulfide dissolution, but each is an imperfect solution at best. Therefore, an improved acid well treatment fluid that is relatively inexpensive, has low corrosivity and effectively dissolves FeS without significant liberation of H.sub.2 S is a long-sought after and highly desirable goal.